U.S. Pat. No. 4,444,577 suggests a method for removing carbon dioxide, ethane and heavier components from a high pressure natural gas stream comprising methane, carbon dioxide, ethane and heavier components, which method comprises: (a) partly condensing and expanding the high pressure natural gas stream to obtain a stream enriched in methane and carbon dioxide and at least one liquid stream enriched in ethane and heavier components; (b) introducing the stream enriched in methane and carbon dioxide at fractionation pressure into a fractionation column which includes a plurality of fractionation stages; (c) introducing the liquid stream(s) at fractionation pressure into the fractionation column at a level which is below the level of introducing the stream enriched in methane and carbon dioxide; (d) removing from the fractionation column, below the level of introducing the liquid stream(s), at least one fluid stream, heating the fluid stream(s) to obtain reboiling stream(s), and introducing the reboiling stream(s) into the fractionation column; (e) removing from the bottom of the fractionation column a liquid stream enriched in carbon dioxide, ethane and heavier components; and (f) removing from the top of the fractionation column a gaseous overhead stream enriched in methane.
In the known method the gaseous overhead stream is partly condensed to obtain a gaseous product stream which is enriched in methane and a liquid stream which is introduced as reflux into the top of the fractionation column. Partly condensing the gaseous overhead stream is done by expanding the gaseous overhead stream to a low pressure in an expansion engine in the form of a turbine expander. Downstream of the turbine expander, the expanded and cooled overhead stream is separated into a gaseous product stream and a liquid stream, and this liquid stream is introduced into the top of the fractionation column as a reflux stream. By expanding the gaseous overhead stream, the overhead stream is sufficiently cooled to achieve condensation of ethane and heavier components which were entrained in the gaseous overhead stream. The expansion is done to a pressure below the fractionation pressure prevailing in the fractionation column, and therefore the reflux stream has to be pumped to the fractionation column.
The known method had been developed to treat natural gas containing carbon dioxide to about 0.6 mol percent, so that no upstream carbon dioxide removal step is required.
Applicant had found that when the natural gas contains a larger amount of carbon dioxide, carbon dioxide in the turbine expander can freeze and this freezing will cause operational problems.